text: //
  //  Copyright 2019 The Abseil Authors.
  //
  // Licensed under the Apache License, Version 2.0 (the "License");
  // you may not use this file except in compliance with the License.
  // You may obtain a copy of the License at
  //
  //      https://www.apache.org/licenses/LICENSE-2.0
  //
  // Unless required by applicable law or agreed to in writing, software
  // distributed under the License is distributed on an "AS IS" BASIS,
  // WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
  // See the License for the specific language governing permissions and
  // limitations under the License.
  //
  // -----------------------------------------------------------------------------
  // File: flag.h
  // -----------------------------------------------------------------------------
  //
  // This header file defines the `absl::Flag<T>` type for holding command-line
  // flag data, and abstractions to create, get and set such flag data.
  //
  // It is important to note that this type is **unspecified** (an implementation
  // detail) and you do not construct or manipulate actual `absl::Flag<T>`
  // instances. Instead, you define and declare flags using the
  // `ABSL_FLAG()` and `ABSL_DECLARE_FLAG()` macros, and get and set flag values
  // using the `absl::GetFlag()` and `absl::SetFlag()` functions.
  
  #ifndef ABSL_FLAGS_FLAG_H_
  #define ABSL_FLAGS_FLAG_H_
  
  #include <string>
  #include <type_traits>
  
  #include "absl/base/attributes.h"
  #include "absl/base/config.h"
  #include "absl/base/optimization.h"
  #include "absl/flags/config.h"
  #include "absl/flags/internal/flag.h"
  #include "absl/flags/internal/registry.h"
  #include "absl/strings/string_view.h"
  
  namespace absl {
  ABSL_NAMESPACE_BEGIN
  
  // Flag
  //
  // An `absl::Flag` holds a command-line flag value, providing a runtime
  // parameter to a binary. Such flags should be defined in the global namespace
  // and (preferably) in the module containing the binary's `main()` function.
  //
  // You should not construct and cannot use the `absl::Flag` type directly;
  // instead, you should declare flags using the `ABSL_DECLARE_FLAG()` macro
  // within a header file, and define your flag using `ABSL_FLAG()` within your
  // header's associated `.cc` file. Such flags will be named `FLAGS_name`.
  //
  // Example:
  //
  //    .h file
  //
  //      // Declares usage of a flag named "FLAGS_count"
  //      ABSL_DECLARE_FLAG(int, count);
  //
  //    .cc file
  //
  //      // Defines a flag named "FLAGS_count" with a default `int` value of 0.
  //      ABSL_FLAG(int, count, 0, "Count of items to process");
  //
  // No public methods of `absl::Flag<T>` are part of the Abseil Flags API.
  //
  // For type support of Abseil Flags, see the marshalling.h header file, which
  // discusses supported standard types, optional flags, and additional Abseil
  // type support.
  #if !defined(_MSC_VER) || defined(__clang__)
  template <typename T>
  using Flag = flags_internal::Flag<T>;
  #else
  #include "absl/flags/internal/flag_msvc.inc"
  #endif
  
  // GetFlag()
  //
  // Returns the value (of type `T`) of an `absl::Flag<T>` instance, by value. Do
  // not construct an `absl::Flag<T>` directly and call `absl::GetFlag()`;
  // instead, refer to flag's constructed variable name (e.g. `FLAGS_name`).
  // Because this function returns by value and not by reference, it is
  // thread-safe, but note that the operation may be expensive; as a result, avoid
  // `absl::GetFlag()` within any tight loops.
  //
  // Example:
  //
  //   // FLAGS_count is a Flag of type `int`
  //   int my_count = absl::GetFlag(FLAGS_count);
  //
  //   // FLAGS_firstname is a Flag of type `std::string`
  //   std::string first_name = absl::GetFlag(FLAGS_firstname);
  template <typename T>
  ABSL_MUST_USE_RESULT T GetFlag(const absl::Flag<T>& flag) {
    return flags_internal::FlagImplPeer::InvokeGet<T>(flag);
  }
  
  // SetFlag()
  //
  // Sets the value of an `absl::Flag` to the value `v`. Do not construct an
  // `absl::Flag<T>` directly and call `absl::SetFlag()`; instead, use the
  // flag's variable name (e.g. `FLAGS_name`). This function is
  // thread-safe, but is potentially expensive. Avoid setting flags in general,
  // but especially within performance-critical code.
  template <typename T>
  void SetFlag(absl::Flag<T>* flag, const T& v) {
    flags_internal::FlagImplPeer::InvokeSet(*flag, v);
  }
  
  // Overload of `SetFlag()` to allow callers to pass in a value that is
  // convertible to `T`. E.g., use this overload to pass a "const char*" when `T`
  // is `std::string`.
  template <typename T, typename V>
  void SetFlag(absl::Flag<T>* flag, const V& v) {
    T value(v);
    flags_internal::FlagImplPeer::InvokeSet(*flag, value);
  }
  
  // GetFlagReflectionHandle()
  //
  // Returns the reflection handle corresponding to specified Abseil Flag
  // instance. Use this handle to access flag's reflection information, like name,
  // location, default value etc.
  //
  // Example:
  //
  //   std::string = absl::GetFlagReflectionHandle(FLAGS_count).DefaultValue();
  
  template <typename T>
  const CommandLineFlag& GetFlagReflectionHandle(const absl::Flag<T>& f) {
    return flags_internal::FlagImplPeer::InvokeReflect(f);
  }
  
  ABSL_NAMESPACE_END
  }  // namespace absl
  
  
  // ABSL_FLAG()
  //
  // This macro defines an `absl::Flag<T>` instance of a specified type `T`:
  //
  //   ABSL_FLAG(T, name, default_value, help);
  //
  // where:
  //
  //   * `T` is a supported flag type (see the list of types in `marshalling.h`),
  //   * `name` designates the name of the flag (as a global variable
  //     `FLAGS_name`),
  //   * `default_value` is an expression holding the default value for this flag
  //     (which must be implicitly convertible to `T`),
  //   * `help` is the help text, which can also be an expression.
  //
  // This macro expands to a flag named 'FLAGS_name' of type 'T':
  //
  //   absl::Flag<T> FLAGS_name = ...;
  //
  // Note that all such instances are created as global variables.
  //
  // For `ABSL_FLAG()` values that you wish to expose to other translation units,
  // it is recommended to define those flags within the `.cc` file associated with
  // the header where the flag is declared.
  //
  // Note: do not construct objects of type `absl::Flag<T>` directly. Only use the
  // `ABSL_FLAG()` macro for such construction.
  #define ABSL_FLAG(Type, name, default_value, help) \
    ABSL_FLAG_IMPL(Type, name, default_value, help)
  
  // ABSL_FLAG().OnUpdate()
  //
  // Defines a flag of type `T` with a callback attached:
  //
  //   ABSL_FLAG(T, name, default_value, help).OnUpdate(callback);
  //
  // `callback` should be convertible to `void (*)()`.
  //
  // After any setting of the flag value, the callback will be called at least
  // once. A rapid sequence of changes may be merged together into the same
  // callback. No concurrent calls to the callback will be made for the same
  // flag. Callbacks are allowed to read the current value of the flag but must
  // not mutate that flag.
  //
  // The update mechanism guarantees "eventual consistency"; if the callback
  // derives an auxiliary data structure from the flag value, it is guaranteed
  // that eventually the flag value and the derived data structure will be
  // consistent.
  //
  // Note: ABSL_FLAG.OnUpdate() does not have a public definition. Hence, this
  // comment serves as its API documentation.
  
  // -----------------------------------------------------------------------------
  // Implementation details below this section
  // -----------------------------------------------------------------------------
  
  // ABSL_FLAG_IMPL macro definition conditional on ABSL_FLAGS_STRIP_NAMES
  #if !defined(_MSC_VER) || defined(__clang__)
  #define ABSL_FLAG_IMPL_FLAG_PTR(flag) flag
  #define ABSL_FLAG_IMPL_HELP_ARG(name)                      \
    absl::flags_internal::HelpArg<AbslFlagHelpGenFor##name>( \
        FLAGS_help_storage_##name)
  #define ABSL_FLAG_IMPL_DEFAULT_ARG(Type, name) \
    absl::flags_internal::DefaultArg<Type, AbslFlagDefaultGenFor##name>(0)
  #else
  #define ABSL_FLAG_IMPL_FLAG_PTR(flag) flag.GetImpl()
  #define ABSL_FLAG_IMPL_HELP_ARG(name) &AbslFlagHelpGenFor##name::NonConst
  #define ABSL_FLAG_IMPL_DEFAULT_ARG(Type, name) &AbslFlagDefaultGenFor##name::Gen
  #endif
  
  #if ABSL_FLAGS_STRIP_NAMES
  #define ABSL_FLAG_IMPL_FLAGNAME(txt) ""
  #define ABSL_FLAG_IMPL_FILENAME() ""
  #define ABSL_FLAG_IMPL_REGISTRAR(T, flag)                                      \
    absl::flags_internal::FlagRegistrar<T, false>(ABSL_FLAG_IMPL_FLAG_PTR(flag), \
                                                  nullptr)
  #else
  #define ABSL_FLAG_IMPL_FLAGNAME(txt) txt
  #define ABSL_FLAG_IMPL_FILENAME() __FILE__
  #define ABSL_FLAG_IMPL_REGISTRAR(T, flag)                                     \
    absl::flags_internal::FlagRegistrar<T, true>(ABSL_FLAG_IMPL_FLAG_PTR(flag), \
                                                 __FILE__)
  #endif
  
  // ABSL_FLAG_IMPL macro definition conditional on ABSL_FLAGS_STRIP_HELP
  
  #if ABSL_FLAGS_STRIP_HELP
  #define ABSL_FLAG_IMPL_FLAGHELP(txt) absl::flags_internal::kStrippedFlagHelp
  #else
  #define ABSL_FLAG_IMPL_FLAGHELP(txt) txt
  #endif
  
  // AbslFlagHelpGenFor##name is used to encapsulate both immediate (method Const)
  // and lazy (method NonConst) evaluation of help message expression. We choose
  // between the two via the call to HelpArg in absl::Flag instantiation below.
  // If help message expression is constexpr evaluable compiler will optimize
  // away this whole struct.
  // TODO(rogeeff): place these generated structs into local namespace and apply
  // ABSL_INTERNAL_UNIQUE_SHORT_NAME.
  // TODO(rogeeff): Apply __attribute__((nodebug)) to FLAGS_help_storage_##name
  #define ABSL_FLAG_IMPL_DECLARE_HELP_WRAPPER(name, txt)                       \
    struct AbslFlagHelpGenFor##name {                                          \
      /* The expression is run in the caller as part of the   */               \
      /* default value argument. That keeps temporaries alive */               \
      /* long enough for NonConst to work correctly.          */               \
      static constexpr absl::string_view Value(                                \
          absl::string_view absl_flag_help = ABSL_FLAG_IMPL_FLAGHELP(txt)) {   \
        return absl_flag_help;                                                 \
      }                                                                        \
      static std::string NonConst() { return std::string(Value()); }           \
    };                                                                         \
    constexpr auto FLAGS_help_storage_##name ABSL_INTERNAL_UNIQUE_SMALL_NAME() \
        ABSL_ATTRIBUTE_SECTION_VARIABLE(flags_help_cold) =                     \
            absl::flags_internal::HelpStringAsArray<AbslFlagHelpGenFor##name>( \
                0);
  
  #define ABSL_FLAG_IMPL_DECLARE_DEF_VAL_WRAPPER(name, Type, default_value)     \
    struct AbslFlagDefaultGenFor##name {                                        \
      Type value = absl::flags_internal::InitDefaultValue<Type>(default_value); \
      static void Gen(void* absl_flag_default_loc) {                            \
        new (absl_flag_default_loc) Type(AbslFlagDefaultGenFor##name{}.value);  \
      }                                                                         \
    };
  
  // ABSL_FLAG_IMPL
  //
  // Note: Name of registrar object is not arbitrary. It is used to "grab"
  // global name for FLAGS_no<flag_name> symbol, thus preventing the possibility
  // of defining two flags with names foo and nofoo.
  #define ABSL_FLAG_IMPL(Type, name, default_value, help)                       \
    extern ::absl::Flag<Type> FLAGS_##name;                                     \
    namespace absl /* block flags in namespaces */ {}                           \
    ABSL_FLAG_IMPL_DECLARE_DEF_VAL_WRAPPER(name, Type, default_value)           \
    ABSL_FLAG_IMPL_DECLARE_HELP_WRAPPER(name, help)                             \
    ABSL_CONST_INIT absl::Flag<Type> FLAGS_##name{                              \
        ABSL_FLAG_IMPL_FLAGNAME(#name), ABSL_FLAG_IMPL_FILENAME(),              \
        ABSL_FLAG_IMPL_HELP_ARG(name), ABSL_FLAG_IMPL_DEFAULT_ARG(Type, name)}; \
    extern absl::flags_internal::FlagRegistrarEmpty FLAGS_no##name;             \
    absl::flags_internal::FlagRegistrarEmpty FLAGS_no##name =                   \
        ABSL_FLAG_IMPL_REGISTRAR(Type, FLAGS_##name)
  
  // ABSL_RETIRED_FLAG
  //
  // Designates the flag (which is usually pre-existing) as "retired." A retired
  // flag is a flag that is now unused by the program, but may still be passed on
  // the command line, usually by production scripts. A retired flag is ignored
  // and code can't access it at runtime.
  //
  // This macro registers a retired flag with given name and type, with a name
  // identical to the name of the original flag you are retiring. The retired
  // flag's type can change over time, so that you can retire code to support a
  // custom flag type.
  //
  // This macro has the same signature as `ABSL_FLAG`. To retire a flag, simply
  // replace an `ABSL_FLAG` definition with `ABSL_RETIRED_FLAG`, leaving the
  // arguments unchanged (unless of course you actually want to retire the flag
  // type at this time as well).
  //
  // `default_value` is only used as a double check on the type. `explanation` is
  // unused.
  // TODO(rogeeff): replace RETIRED_FLAGS with FLAGS once forward declarations of
  // retired flags are cleaned up.
  #define ABSL_RETIRED_FLAG(type, name, default_value, explanation)      \
    static absl::flags_internal::RetiredFlag<type> RETIRED_FLAGS_##name; \
    ABSL_ATTRIBUTE_UNUSED static const auto RETIRED_FLAGS_REG_##name =   \
        (RETIRED_FLAGS_##name.Retire(#name),                             \
         ::absl::flags_internal::FlagRegistrarEmpty{})
  
  #endif  // ABSL_FLAGS_FLAG_H_
  